A known inflator stores gas for inflating an inflatable vehicle occupant restraint, such as an air bag. The gas typically comprises a mixture of at least two different gases. The gas is directed into the air bag to inflate the air bag to help protect the vehicle occupant from a forceful impact with parts of the vehicle on sudden vehicle deceleration. It is desirable to be able to detect gas leakage from the inflator over the life of the inflator, since a reduction in the amount of gas stored in the inflator could affect inflation of the air bag.